1. Field of the Invention
The present invention relates to measuring devices, and in particular, though not exclusively, measuring devices for use in an exposure apparatus and to a wafer alignment device.
2. Description of the Related Art
In order to fabricate fine semiconductor elements, such as a semiconductor memory and logic circuits, a reduction projection exposure process using a UV ray has been performed.
The minimum object dimension to be transferred by the reduction projection exposure is proportional to the wavelength of light used for transferring, and is inversely proportional to the numerical aperture of a projection optical system. Hence, the reduction in wavelength of a light ray for used in transferring fine circuit patterns has been promoted, so that wavelengths of used UV rays have been reduced, such as a mercury lamp i-ray (wavelength 365 nm), KrF excimer laser (248 nm), and ArF excimer laser (193 nm).
However, with the rapid miniaturization of semiconductor elements, the size needed has resulted in a limit in ability of lithography using UV light. Thus, for efficiently exposing circuit patterns as small as 42 nm in size, exposure apparatus's have been proposed using an EUV (extreme ultra violet) ray with a wavelength of 10 to 15 nm, which is further smaller than that of the UV ray.
Since a light ray with a wavelength of 10 to 15 nm is very largely absorbed by a substance, lens optical systems using visible light or UV light, typically have light refraction levels that are unusable, thus reflection optical systems are used in exposure apparatuses using the EUV ray. In this case, a reflection-type reticle having a pattern to be transferred formed on a mirror with an absorber is also used.
Furthermore, since the EUV ray is generally absorbed in the atmosphere and attenuated, it needs to be used under vacuum. Thus, the exposure apparatus includes a vacuum chamber evacuated inside. In such an exposure apparatus, use of electronic instruments arranged inside the vacuum chamber is limited. For example, an electronic instrument can malfunction under the vacuum or solder used in an electric circuit can emit pollutant.
Japanese patent Laid-Open No. 2001-217191 discuses an exposure apparatus, where parts of elements of an alignment system for aligning a wafer to a mask is arranged outside the vacuum chamber. The part includes a laser module and a photo-detector. When doing so, the configuration of the apparatus is largely limited. In general, vacuum-enabled measuring devices are inferior in accuracy than normal measuring devices, so that to use these devices is inappropriate.